Ferromagnetic materials, such as iron, nickel, steel and other materials, are used to make many items, such as pipes, beams and ocean vessel hulls. As used herein, “ferromagnetic material” includes both ferromagnetic and ferrimagnetic material. In many cases, these materials are subject to corrosion and/or erosion. As used herein, corrosion means loss of material as a result of chemical reaction, most commonly oxidation. As used herein, erosion means loss of material as a result of a mechanical process, such as abrasion. For example, sand produced in an oil or gas well can abrade the inside of a pipeline carrying oil or gas from the well. Material loss due to corrosion and/or erosion is collectively referred to herein as a “defect.” As used herein, the term defect also includes a crack, or a void or inclusion of foreign material, such as might occur during manufacture or later. If allowed to occur beyond a critical point, corrosion or erosion may compromise structural integrity of an item, possibly resulting in a catastrophic failure, such as an oil spill, building collapse or ship sinking.
Various apparatus and methods have been used in the prior art in attempts to detect defects in ferromagnetic materials and items made of ferromagnetic materials. Some of these apparatus and methods require removing thermal insulation and striping off corrosion inhibiting surface treatments to gain direct access to a surface of the ferromagnetic material. In some cases, the surface must be polished to create a pristine interface to a sensor or wave propagation from the sensor. These steps are costly, time-consuming and often compromise the thermal insulation and/or the surface treatments.
All prior art apparatus and methods for detecting defects in ferromagnetic materials known to the inventors involve introducing energy into the material. For example, acoustic sensors send a sound wave into the material and measure the signal that returns. Guided-wave and topographic sensors similarly send electromagnetic waves into the material and sense reflections or transport times of the wave. In a different view of imparting energy into the item being measured, Rohrback Cosasco Systems, Inc. produces a line of sand erosion detection probes under the tradename “Quicksand.” These probes do not directly measure erosion of pipes, etc. Instead, these probes are sacrificial, in that they detect erosion of portions of the probes themselves. Systems based on such probes assume pipes and other items erode at approximately the same rate as the probes' sacrificial portions. Furthermore, the probes rely on flow of fluid through the pipe, therefore requiring energy to be introduced into the pipe in the form of fluid flow. These systems can detect erosion only inside a pipe. These systems cannot detect defects elsewhere, such as inside the pipe wall or on an outside surface of a pipe, nor can they infer the condition of a pipe due to erosion before the sensor was in place.
Some prior art apparatus and methods involve magnetometry in attempts to detect defects in ferromagnetic materials. For example, U.S. Pat. Nos. 8,542,127 and 8,447,532, both by Valerian Goroshevskiy, et al., disclose using the inverse magnetostrictive Villari effect. The inverse magnetostrictive Villari effect involves changes in a material's magnetic susceptibility under applied mechanical stress. If a pipe suffers a defect, the pipe's magnetic susceptibility when the pipe material is mechanically stressed, for example when the pipe is pressurized, is different than when the pipe is not mechanically stressed. The Goroshevskiy patents rely on detecting this change in magnetic susceptibility as pressure within the pipe changes. Thus, energy must be introduced into the pipe in the form of pressurizing the inferior of the pipe. Some items, such as pipes, remain unused, and therefore unpressurized, for periods of time during which defects may develop. Other structures, such as ship hulls or structural elements, do not lend themselves to known pressurization cycling. However, without pressurization, the Goroshevskiy apparatus and methods cannot detect these defects. Furthermore, Goroshevskiy can determine a defect's location only along the length of a pipe; Goroshevskiy cannot determine the defect's location circumferentially around the pipe.